With the rapid advancement of the laser technologies, laser sources are becoming more compact and efficient. For instance, a diode laser is a popular laser source because of its small size, low-power consumption, and ease of mass production; however, the emission wavelength of a diode laser is limited by material properties or, more specifically, by the quantum energy levels of the laser gain medium. A diode-laser pumped solid-state (DPSS) laser is also playing an important role in various laser applications due to its superior properties in, for example, generating high peak power and good laser-mode profiles. A DPSS laser comprises a laser gain medium absorbing the pump-diode laser energy and a laser cavity resonating the emission wave from the laser gain medium. In such a configuration, lasers of different wavelengths can be generated by using different laser gain media in suitable laser resonators. Since the laser wavelength is fixed to the available energy levels of a laser material, the wavelength of a DPSS laser employing a certain laser gain medium can not be arbitrarily tuned.
A nonlinear optical process allows laser frequency mixing to generate new laser frequencies or wavelengths that are not generally available from the quantum energy levels of a laser material. Therefore, a wavelength-tunable coherent light source can be implemented by installing a nonlinear optical material inside or outside a laser cavity. Second-order nonlinear wavelength conversion utilizes the second-order (χ(2)) nonlinear susceptibility of a nonlinear optical material and is usually an easier process compared with a third-order nonlinear wavelength conversion process. Among the second-order nonlinear wavelength-conversion processes, for example, an optical parametric process can provide broad laser-wavelength tuning. In χ(2)-based nonlinear wavelength conversion, phase-matching among mixing waves is required, and is often achieved in a birefringence nonlinear-optical material with carefully arranged polarization and propagation directions of the mixing optical waves. Such a stringent phase-matching requirement usually sacrifices the largest available nonlinear coupling coefficient in a given nonlinear optical material and limits the energy conversion efficiency of laser wavelength conversion. In recent years, the so-called quasi-phase matching (QPM) technique has removed the aforementioned limitation by compensating the phase mismatch of the mixing waves in a nonlinear optical material by using a spatially modulated nonlinear coefficient. Such a QPM technique allows a laser-wavelength-conversion process to access the maximum nonlinear coefficient of a nonlinear optical material and thus to obtain much higher wavelength-conversion efficiency.
Many important laser applications require high peak laser power with a short laser pulse width. In particular, a high laser power can greatly increases the conversion efficiency of nonlinear laser-wavelength conversion. Laser Q-switching is a common way of obtaining a high peak laser power from a laser source.
Q-switching is a popular scheme for generating nanosecond and high-peak-power laser radiations. The working principle of a Q-switched laser is based on a technique, with which the laser energy is accumulated in a time period comparable to the upper-level lifetime of the laser gain medium and is released in a short period of time to generate the high-power laser pulse. During the laser energy storage, the laser cavity is kept in a high-loss or a low-Q state. A fast switching from the low-Q to a low-loss or a high-Q state for the laser cavity to release the stored energy in a short laser pulse. In general, there are two laser Q-switching schemes, active Q-switching and passive Q-switching Compared with a passively Q-switched laser, an actively Q-switched laser is advantageous in handling a wider range of laser power and in controlling the timing of the generated laser pulses. Usually an actively Q-switched laser employs an acousto-optic (AO) Q-switch or an electro-optic (EO) Q-switch. An AO Q-switch requires a radio-frequency (RF) voltage driver and an EO Q-switch requires a pulsed high-voltage (in the kV range) driver. An AO Q-switch is usually a Bragg cell that deflects a light wave according to the Bragg deflection condition from an acousto-optic grating and can be fairly insensitive to laser's polarization. On the other hand, an EO Q-switch is usually a Pockels cell that utilizes a voltage pulse to control the polarization loss and thus the quality factor (the Q-factor) of a laser cavity. For fast laser Q-switching, EO switching is the preferred scheme due to its much faster response from the EO effect of an EO crystal.
The present invention is related to an EO Bragg deflector comprising an electrode-coated EO material with a spatially modulated EO coefficient forming a grating when an electric field is applied to the material. In particular, the present invention employs this EO Bragg deflector as a laser Q-switch that does not require a RF voltage driver, has a much lower Q-switch voltage than that of a conventional EO Q-switch based on polarization-loss control and using materials such as potassium dihydrogen phosphate (KDP), potassium titanyl phosphate (KTP), lithium niobate (LN), etc. Thus the present invention allows a compact and low-cost design for an actively Q-switched laser system. Since both an EO Bragg deflector and a QPM wavelength converter have spatially modulated χ(2) nonlinear coefficients in the material, the EO Bragg deflector of the present invention can be easily integrated to a QPM nonlinear wavelength converter to perform simultaneous laser Q-switching and wavelength conversion for a laser source. The Q-switched wavelength-conversion laser is particularly simple, compact, and efficient, if the EO Bragg deflector of the present invention and the QPM laser-wavelength converter are integrated into a monolithic nonlinear-optical-material substrate in a single fabrication process.
To alleviate the drawbacks in the prior arts, the applicant carried out a major research-and-development effort to conceive an EO Bragg deflector and a method of using it as a laser Q-switch in an actively Q-switched laser and in an actively Q-switched wavelength-conversion laser.